The invention is a method and device for monitoring occurrence of a back-up or a jamming of containers on a conveyor which conveys the containers by frictional contact between the conveyor and containers.
So called jam or back-up sensors are applied to conveyor or treatment systems for containers for a variety of monitoring and control functions. The purpose is to signal a speed increase and open the container gate when a back-up at its infeed conveyor occurs and to signal a speed decrease and close the container gate when a back-up occurs at its discharge side as in German Patent No. 1,803,332.
One known type of jam or back-up sensing device detects container offset or deflection from the normal line of travel in a control zone which offset results from pressure between containers flowing in a line. The offset sensor is connected to a switch which converts a mechanical force to an electric signal. Such a system is described in German Patent Specification No. 922,517 . This device can only be applied where the conveyed containers have a special shape such as bulbous midsections which are subject to lateral shifting from the line of movement when there is a blockage. It cannot be used for containers with parallel walls such as special shaped square bottles. In its function, it depends largely on the coefficient of friction between the conveying medium and the container, the container weight, the type of chain lubrication or the condition of the conveyor and reacts only after the line pressure or slippage between the conveyor and the containers has exceeded a certain value. This known type of jam switch reacts relatively slow and imprecisely. The bottle offset in the sensing region, furthermore, causes undesirable lateral movement of the containers which results in an increased noise level, especially where glass containers are involved.
Another known device for signaling a back-up is especially for cases where the containers are conveyed in a single lane as in German Laid Open Patent Specification No. 1,261,348. This system uses two photocells with crossing light beams and a processing unit. One of the two photocells responds to a beam at right angles to the container flow and is used to count the containers passing the control zone within a given time so it provides an indications of the existing conveying rate. The second photocell, mounted at an acute angle to the container flow, checks for the presence of containers in the control zone. If the first photocell does not emit a counting pulse, although the second cells sees containers in the area of the control zone, a back-up signal is emitted. Thus, this back-up signal is emitted when the containers are at a complete standstill regardless of whether the conveyor continues to run or not. The system does not differentiate between a real back-up where the conveyor continues to run and a complete stop of the conveyor. Further, this device does not produce a back-up signal when the conveyor chain slippage is more or less under the containers as long as the containers pass the control point. In this case, the shift register connected to the first photocell interprets the signal as "low speed". The described condition, where the conveyor slips under the containers, frequently occurs on all conveying or packaging system and cannot, therefore, be detected with a known device under discussion. The device does not meet the desired objectives.